1. Field of the Invention
The present invention relates to once-through boilers and, more particularly, is concerned with improved steam temperature control for once-through boilers.
2. Background of the Invention
In a commercial once-through boiler, heat, from which steam and, ultimately, electricity are generated, is produced by burning a fossil fuel, such as coal or oil. The boiler is connected to a steam turbine which in conjunction with a generator produces electricity. The once through boiler itself includes a vessel body. A heater is attached to an interior surface of the vessel body for heating any water or steam passing therethrough. In a simplified form, a series of contiguous components pass through the vessel and forms a continuous conduit for conveying water and, eventually, steam therethrough. The conduit extends generally from the vessel exterior into the vessel and then out of the vessel again. Once the conduit enters the vessel, the water flowing therethrough is converted to steam in a first portion of the conduit, and a latter portion (i.e., superheater) of the conduit heats the steam before it exits the vessel via an outlet. To control the temperature of the steam discharged from the vessel, the temperature of the superheater is varied. A spray valve positioned outside the vessel discharges water into the conduit to counteract the temperature rise associated with the heater, if necessary. The temperature of the superheater is controlled by cooperative interaction between the heater and the spray valve. For example, to increase the heating in the superheater, the heater temperature is increased thereby increasing the temperature rise within the superheater. To lower the temperature of the superheater, the spray valve sprays water into the superheater thereby counteracting the heat of the heater and reducing the temperature rise within the superheater.
A thermocouple is attached to the conduit at the vessel outlet (outside the vessel body), and it communicates with both the spray valve and the heater. This thermocouple measures the temperature of the steam passing therethrough. For efficiency, the steam leaving the boiler and entering the turbine should be within preset limits. The thermocouple measures the temperature to determine if the preset limits are met and communicates this information to the heater and spray valve. If the temperature is below the bottom acceptable limit, the heater generates more heat, further heating the steam in the superheater. If the temperature exceeds an acceptable upper limit, the spray valve releases water into the superheater. This action lowers the temperature rise within the superheater so that heating of the steam is reduced.
Although the presently utilized system is efficient, it is not without drawbacks. In this regard, the steam has significant travel time through the superheater. If the temperature of the steam traveling through the superheater is not within the predetermined limits, it will not be detected until it reaches the thermocouple adjacent the steam outlet. The thermocouple will communicate this information to the spray valve and heater which, after receiving this information, reacts appropriately to either raise or lower the temperature of the steam within the superheater. This is a drawback because a time delay exists before efficient steam is passed to the turbine.
Consequently, a need exists for an improved temperature control of steam developed by once-through boilers.